After the initial cutscene and you gain control of Link, walk outside. Go down the ladder and talk to Sara, who is happy that you finally got a fairy. Before you can go to meet the Great Deku Tree, you will need to acquire a few times, since the Kokiri blocking the exit won't let you pass unless you have a sword and shield.
THE KOKIRI SWORD
Go to your left and up the hill, where you'll find some fences that lead to a small hole. Crawl through the hole to appear in a separate area, where there's a boulder rolling around. You can get 10 Rupees here from two separate Blue Rupees (5 each), by going right when you first get in the area and in the little alcove near where the boulder is rolling; you will need some later. Wait for the boulder to pass, then open the chest to gain the Kokiri Sword. Equip it, then exit the area via the small hole you used to enter it.
THE DEKU SHIELD
You have a sword, but now you need a shield. The item shop inside of the village sells one, but it costs 40 rupees. When you exit via the small hole, talk to the NPC to the right, who tells you how to perform some maneuvers. Target the rock, then backflip a couple of times to get some invisible Rupees (6 of them; around 16-18 total).
Slash the bushes to try and get some more, then read the sign farther away from you and perform the thrust attack to make another Blue Rupee appear (21-23 total). The house straight across from you has a Blue Rupee behind it, and four chests inside with more Rupees (36-38 total). Find the house with the ramp leading to the top, then cross the wooden bridges to find another Blue Rupee (41-43 total).
You can get some more by hopping across the platforms in the water one way, then back the other. Once you have enough, approach the shop and you will get a lesson in targeting. When that's all over, head inside and purchase the Deku Shield. Equip it, then head over to the exit where the one NPC was blocking you previously, who should let you through now.
On the path to the Great Deku Tree, you'll encounter some Deku Babas, which will stick straight up. Either target them and slash them, or just slash them normally. They will most likely drop Deku Sticks, which will be helpful in the upcoming dungeon. Go along the rest of the path to come face to face with the Great Deku Tree, which after a long conversation, will open up a door that leads inside, to your first dungeon.
